This invention relates to the field of aeronautics and more specifically to enhancing the efficiency and performance of air vehicles.
The National Aeronautics and Space Administration or NASA is a United States government agency that is responsible for science and technology related to air and space. When NASA started in 1958, it began a program of human spaceflight. The Mercury, Gemini, and Apollo programs helped NASA learn about flying in space and resulted in the first human landing on the moon in 1969. NASA has helped develop and test a variety of cutting-edge aircraft. These aircraft include planes that have set new records. Among other benefits, these tests have helped engineers improve air transportation. NASA technology has contributed to many items used in everyday life, from smoke detectors to medical tests.
There is an increasing global awareness of greenhouse gas emission due to ever-increasing fossil fuel consumption in many sectors of the global economy. The transportation sector is one of leading contributors to the environmentally harmful emission and the escalating fuel consumption. Thus, it is realized that solutions to this global challenge must be developed through engineering innovations in all economic activities. To date, the ground transportation sector for consumers has witnessed many new energy-efficient technologies developed by the automotive industry, such as, hybrid vehicles and all-electric vehicles. In this same context for air transportation, NASA is taking a lead role in developing a “green” aviation initiative that seeks technology development for environmentally responsible future aviation systems to meet national and global challenges of improving aircraft fuel efficiency while reducing noise and emissions. NASA leadership in this area is important to maintain its preeminent heritage and future leadership in aeronautics.
Green aviation focuses on new aeronautic technologies that may potentially revolutionize aviation systems that may lead to improved aerodynamic efficiency, less fuel burn, and reduced noise and emissions. These important goals represent current challenges in the present aviation systems in response to the emerging needs for innovative aircraft design that may address future aviation systems. In the context of commercial aviation, civilian aircraft remains the largest U.S. export category ($9.4 billion, “U.S. Export Fact Sheet, March 2009).
Therefore, the increasing demand for fuel-efficient aircraft for global commerce prompts the aircraft industry to address improved fuel efficiency as a top national and global challenge. Air transportation is projected to increase rapidly in the future. As a major source of fossil fuel consumption, any small increase in aircraft's aerodynamic efficiency may translate into significant cost savings for the air transportation industry. Aerodynamic efficiency, which is defined as the ratio of lift to drag, is one of the most important considerations in aircraft design. To achieve aerodynamic efficiency, aircraft designers conduct detail aerodynamic design and analysis of the geometry of an aircraft in order maintain aircraft drag to a minimum.
Typically, a major source of aircraft drag is derived from the aircraft wing. In flight, the wing provides most of the lift force to balance the aircraft weight as it is airborne. As the wing generates lift, it also generates a source of drag known as induced drag. This drag source is dependent on the wing lift. A parabolic relationship between the wing lift coefficient CL and the drag coefficient CD, also referred to as a drag polar, is usually employed in aerodynamic analysis as follows:CD=CD0+CL2/πARε
The quantity ε is called the span (or Oswald's) efficiency factor. When the lift distribution is most optimal over a wing, this factor is equal to unity. When the wing lift is less than optimal, it is less than unity. A typical value may be between 0.8 and 0.9 for a very efficient wing design. Thus, aircraft drag may be reduced by having an aerodynamically efficient wing design with the span efficiency factor as close to unity as possible.
Yet, another indirect way of reducing drag is to reduce aircraft weight. As the aircraft weight is reduced, the wing lift is also reduced as the aircraft weight is always in balance with the wing lift in steady-state flight. The drag polar reveals that as the wing lift coefficient decreases, the drag coefficient also decreases quadratically. Modern aircraft is designed to achieve as much weight savings as possible in order to realize this important objective.
One aspect of weight savings is accomplished by employing lightweight advanced engineered materials in aircraft structures. An important aircraft structure is the aircraft wing. As a consequence of the use of lightweight materials in aircraft structure, aircraft wings become much more flexible. The main objective of a lightweight airframe design is to reduce the wing lift requirement which in turn reduces drag. However, as the aircraft wings become flexible, potential adverse impacts on aerodynamic efficiency may exist.
An aircraft wing is normally designed for optimal aerodynamic performance at only one point in a flight envelope by tailoring a wing lift distribution to achieve the best span efficiency possible. This design point is usually at the mid-point in a design cruise range, typically when the fuel stored inside aircraft wings is half-way spent. Due to the effect of aeroelasticity (structural interactions with aerodynamics), aircraft wings tend to deflect in flight by exhibiting a combined bending and twisting motion. If the wings become much more flexible due to the use of lightweight materials in the construction, the deflection may change the optimal wing shape which in turn changes the optimal wing lift distribution. Consequently, the span efficiency may decrease as the aircraft wings deflect significantly from the optimal wing shape at the design point. This reduction in span efficiency therefore causes an increase in drag which may potentially negate the original goal of employing lightweight airframe design for drag reduction.
Thus, it is realized that there is an unfulfilled need to have an ability to improve aerodynamic efficiency and reduce drag of aircraft due to non-optimal wing shape in flight. More specifically, when an aircraft's optimal wing shape is deviated from its design shape due to any reasons such as wing flexibility, or when an aircraft wing is not at its optimal aerodynamic performance such as high lift requirements at any point inside a flight envelope, what would be needed is an ability to alter the wing shape in a such a manner as to increase the aerodynamic efficiency and the aerodynamic performance at any point in a flight envelope. This ability would achieve a drag reduction objective while enabling aircraft wings to be adaptable to any given performance requirements throughout a flight envelope. Furthermore, this ability may be viewed a key enabling feature for lightweight airframe design, for otherwise the drag benefit of a lightweight airframe design could be significantly offset or completely nullified without this ability.